Pressure dependence of the high-frequency light scattering susceptibility of ortho-terphenyl: A mode coupling analysis

The high frequency part of the depolarized light scattering susceptibility has been measured for ortho-terphenyl (OTP) in a broad temperature and pressure range and analyzed using the mode-coupling theory (MCT). We found that the time–temperature–pressure superposition does not work for OTP in the frequency range of the susceptibility minimum and both a and b exponents of the MCT are temperature and pressure dependent. The pressure induced shift of the position of the minimum is much weaker than that of the α-peak resulting in dTc/dP=8.5 K/kbar while dTg/dP=26 K/kbar. Thus any universal scaling of the entire susceptibility including both the α-peak and the MCT minimum is not possible. These results are in contradiction to the previously reported conclusions obtained from the analysis of incoherent neutron scattering data [A. Tölle et al., Phys. Rev. Lett. 80, 2374 (1998)].